This invention relates to a sensor for the measurement of hydrogen content in gas streams. More particularly, it relates to a method for modeling the response time of the sensor.
Industrial uses of hydrogen require a simple and sensitive device for detecting hydrogen leaks and for measuring hydrogen concentrations. Prior art detectors have a long response time to hydrogen. For example, one such detector sold under the trade name Hydran is devoted primarily for the continuous monitoring of slowly variable hydrogen concentrations and has a response time on the order of minutes. Several attempts have been made in the past to improve the response time of hydrogen detectors without much success.
Moreover, known hydrogen detectors failed to consider characteristics influencing the sensor response time. Thus, there is a need for an efficient sensor with a fast response time for analyzing hydrogen content and determining hydrogen partial pressure in gas streams.
Accordingly, the present invention is directed to a micro-fuel cell sensor apparatus and method for the measurement of hydrogen content and hydrogen partial pressure in a gas stream. The sensor is disposed in a fuel-cell housing. The sensor includes a sensing element having first and second gas diffusing electrodes spaced from one another. A fuel-cell spacer having an acidic electrolyte is disposed between the two electrodes. The first electrode is spaced from a first gas permeable membrane by a first cavity, the first membrane being disposed proximate to the housing base.
A second gas permeable membrane is disposed opposite to the first membrane and away from the housing base. Oxygen from atmospheric air is continuously supplied to the second gas diffusing electrode by way of natural diffusion through the second gas permeable membrane. The second electrode is spaced from the second membrane by a second cavity. The amount of oxygen supplied to the second electrode exceeds the amount required for stochiometric reaction with hydrogen diffused through the first membrane.
The above described sensor is disposed in a sensor body having a chamber defined therein for accommodating the sensor. An external gas stream is received in the sensor body via an opening therein. A sensor cover having a recess sealingly mates with the sensor body, the recess in the cover opening into the chamber in the sensor body.
The sensor cover further includes a connector for providing electrical connection to the sensor and also for facilitating measurement of the sensor output. The sensor cover also includes a third gas permeable membrane for supplying oxygen by way of natural diffusion from atmospheric air. Oxygen diffused into the sensor body through the third membrane enters the sensor by way of further diffusion through the second membrane. Excess oxygen may be furnished at the second electrode by an appropriate selection of second and third membranes. The first membrane is chosen to have a high permeability to hydrogen and lower permeability to gases having molecular dimensions that are higher than hydrogen.
In its assembled state, when hydrogen from a gas stream diffuses selectively through the first membrane into the first cavity facing the first gas diffusing electrode, electrochemical charging of the first electrode occurs at a potential corresponding to hydrogen concentration in the first cavity, while the potential of the second electrode remains unchanged. The potential difference created between the first and second electrodes produces a current flow measured by connecting the first and second electrodes through a load resistance. The current measured as a voltage drop across the load resistance represents the micro-fuel cell sensor output.
In one aspect, the present invention thus provides a sensor for measuring partial hydrogen pressure in a gas stream, the sensor including a housing, a sensing element comprising first and second gas diffusing electrodes spaced from one another, a fuel-cell spacer having an acidic electrolyte disposed between the first and second electrodes, a first gas permeable membrane of thickness L and an active surface area A, separating the first electrode from the gas stream by a cavity of volume V, a second gas permeable membrane separating the second electrode from atmospheric air, and a load resistance R connecting the first and second electrodes, wherein a response time T of the sensor is determined by T=aR+b(VL)/A; where xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d are constants. Preferably, the first membrane has higher permeability to hydrogen and lower permeability to gases with molecular dimensions greater than that of hydrogen. The oxygen rate of permeation through the second membrane is higher than hydrogen rate of permeation through the first membrane, whereby oxygen furnished at the second electrode exceeds stochiometric oxygen necessary for the reaction with hydrogen. The first and second electrodes are preferably connected through a load resistance to measure the sensor output.
Oxygen furnished at the second electrode is controlled by an appropriate choice of the second membrane. The first and second membranes are preferably made of a polymeric material. A hydrogen partial pressure gradient is maintained between the first electrode and an external gas stream. The first and second electrodes are preferably identical.
In another aspect, the present invention provides an apparatus for measuring partial hydrogen pressure in a gas stream. The apparatus includes a housing, a micro-fuel cell sensor disposed in the housing, a cover member, the sensor including a sensing element having first and second gas diffusing electrodes spaced from one another, a fuel-cell spacer with an acidic electrolyte interposed between the first and second electrodes, a first gas permeable membrane, of thickness L and an active surface area A, spaced from the first electrode by a cavity of volume V, a second gas permeable membrane spaced from the second electrode to supply oxygen to the second electrode by natural diffusion of atmospheric air, and a load resistance R connecting the first and second electrodes, wherein response time T of the sensor is determined by T=aR+b(VL)/A; where xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d are constants. The cover member includes a connector for providing an electrical connection to the sensor, a third gas permeable membrane disposed in one of the cover member and the housing for receiving atmospheric air. The apparatus further includes means for sealingly attaching the housing to an assembly carrying a gas stream.
In yet another aspect, the present invention provides a method for determining the response time of the micro-fuel cell sensor according to the equation T=(a.R+b.(V.L)/A); where xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d are constants.